The construction of girder bridges - The Design of Prestressed Concrete Bridges - page 464

Figure 15.39 Typical arrangement of casting area
15.8.8 First stage prestress
Once a box segment is complete, it is attached to the previous segment by straight
prestress tendons. These are situated in the top and bottom fl anges, and the centroid
of their force should correspond approximately with the centroid of the cross section,
so the deck is centrally prestressed.
The central spans being effectively built-in, straight prestress will always be re-
centred; that is the prestress parasitic moment Mp will equal the prestress primary
moment Pe , but with the opposite sign ( 6.6 ). Thus this fi rst stage prestress provides a
uniform compression on the section even if its centroid does not lie precisely on the
neutral axis. However, within the leading span of the launched deck, Pe will not be
balanced by Mp , and this may be used to advantage by the designer in defi ning the
additional prestress required.
As the self-weight hogging bending moments are twice the sagging moments, the
ideal deck section properties would give a bottom fi bre modulus that is half the top
fi bre modulus. The tensile bending stresses on the two extreme fi bres would be identical
and the launching prestress would then be minimised.
It is good practice for the tendons to be two or three segments long. This shortens
the programme by reducing the number of tendons to be stressed at each cast, reduces
the bursting stresses on the concrete, reduces the congestion of anchorages, and
improves the economy of the project as the longer the tendons, the lower their unit
cost. The tendons are stressed from live anchors at the rear of the new segment, and
are dead anchored in pockets in the previous segment, Figure 15.40.
Bars are sometimes used for the fi rst stage prestress. They are simple to detail and to
couple, and avoid the necessity for internal anchorage blisters, simplifying the internal
shutter. However they are much more expensive per ton of prestress force, and it is
generally necessary to stress all bars at each construction stage, further increasing the
cost. Their principal advantage would appear to be to the designer rather than to the
contractor or the owner.
The uniform compression on the cross section due to the fi rst stage prestress is in
general benefi cial for the deck in its permanent state, reducing the amount of second
stage prestress required. Consequently, it is generally left in the deck, forming part of
the permanent works. However, if the compressive stresses in service on the bottom